An organoclay is an organophilic cation-modified clay derived from a clay mineral, generally of the smectite group, for example a bentonite, montmorillonite, hectorite, saponite or the like, by replacing the inorganic exchangeable cations, which occur in the natural clay mineral, by organic cations, each comprising at least one hydrocarbon radical which has sufficient carbon atoms to render the surface of the cation-exchanged clay hydrophobic.
Organoclays have been found to be very useful as thixotropic agents in organic media such as, for example, lubricating greases, oil-based drilling fluids, paints, varnishes, enamels and printing inks. However, an organoclay is, in general, difficult to disperse in an organic medium to form the uniform gel structure which is required to thicken, or to increase the viscosity of, the organic medium. Various means have accordingly been used to improve the dispersibility of an organoclay in an organic medium, including the use of a polar organic material, such as a low molecular weight alcohol or ketone, as a dispersing aid, and the use of an excess of a specially selected organic cation in the preparation of the organoclay.
In Finlayson et al, U.S. Pat. No. 4,105,578, an organophilic clay gellant having enhanced dispersibility in organic systems is disclosed which comprises the reaction product of a methyl benzyl dialkyl ammonium compound wherein the compound contains 20 to 35% alkyl groups having 16 carbon atoms, and 60 to 75% alkyl groups having 18 carbon atoms, and a smectite-type clay having a cation exchange capacity of at least 75 milliequivalents per 100 grams of said clay, and wherein the amount of said ammonium compound is from 100 to 120 milliequivalents per 100 grams of said clay, 100% active clay basis.
In Finlayson et al, U.S. Pat. No. 4,412,018, an organophilic clay gellant and methods of using and making the same are disclosed, which comprises the reaction product of an organic cation, an organic anion and a smectite-type clay having a cation exchange capacity of at least 75 milliequivalents per 100 grams of said clay, such that an organic cation-organic anion complex is intercalated with the smectite-type clay and the cation exchange sites of the smectite-type clay are substituted with the organic cation. The examples of the patent illustrate the use of the organophilic clay in thixotropic unsaturated polyester compositions and in thixotropic coating compositions.
In our application, Ser. No. 765,259 filed Aug. 13, 1985, there is described a method which improves the ease of dispersibility of organoclays in organic liquid compositions and avoids the need to provide high shear mixing equipment and the expenditure of the large amounts of energy which are required for the high shear mixing as disclosed in Ser. No. 727,022 filed Apr. 25, 1985.
In particular, this method yields an organoclay which can be dispersed readily into unsaturated polyester solutions. Unsaturated polyester compositions containing various gelling agents are well-known. Additionally, it is known that in the preparation of glass fiber laminates of unsaturated polyester and the like, thixotropic gelling agents are used which allow for the ready mixing and spraying of these resins at high shear rates. At low shear rates, however, these gelling agents greatly increase the viscosity of the polyester, thus avoiding the drainage of the resin on vertical surfaces.
One of the most widely used gelling agents for unsaturated polyesters was asbestos fibers. Asbestos fibers assisted in producing thixotropic polyesters without significantly affecting resin clarity. However, in the past several years, it has become recognized that the use of asbestos under the usual manufacturing conditions is objectionable, because of its long-term adverse effects on health when inhaled. For this reason, the asbestos gelling agents were replaced by a silica aerogel. When about 1% of silica was dispersed in the resin, it provided effective thixotropy. However, such product is comparatively expensive, and because of low bulk density, presents storage and handling problems.
Consequently, the art has turned to the use of organoclays as gelling agents. Conventional organoclays cannot be dispersed in unsaturated polyester solution. They must be predispersed in monomer styrene and the gel subsequently blended with the polyester. This extra stage is expensive and inconvenient. It also limits the amount of organoclay that can be incorporated, to levels which can be physically handled in styrene: above about 8 wt % the gels become unmanageably stiff. Hence there is a need for organoclays which can be stirred directly into polyester solution, like fumed silica.